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Old 12-26-2005, 02:50 PM   #1
beef_bourito
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engine running without exhaust

why do racing engines have ehaust systems? it seems to me as though you wouldn't want an exhaust system. I heard something about the engine needing backpressure but I don't know why.

And what about intake, if you had no intake, just a big tube with one throttle body per cylender, to tuned length. or if you had a diesel with no intake manifold or anything like that, there would be no intake "backpressure".
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Old 12-26-2005, 03:16 PM   #2
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Engines don't NEED backpressure, they use their exaust manifold to suck out more of the exaust gasses. Think about it for a sec just like we are talking about pulses for the intake there are pulses for the exaust. These pulses create vacume behind them, allowing the engine to extract more exaust (assuming they are designed properly) making more room for new unburnt incoming air. Think about it that way. And also the engine does not have to push the old gasses out they are sucked out.
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Old 12-26-2005, 03:20 PM   #3
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Re: engine running without exhaust

well just from an idiots point of view... fire. ever see a race cars exhaust flame during decel? you would burn up or melt everything under the hood of a car if you ran with no kind of exhaust at all. as my shop teacher in high school put it... "even when you let off the gas the carburetor is still carburetin'" that might also contribute to some burned exhaust valves. as for the intake tubes. you would either need 8 little carbs or run a sequential fuel injection. along with throttle plates for each tube and synchronized linkage which would probably need constant adjusting for maximum performance.
you may want to try a google search for exhaust backpressure as it relates to cylinder scavenging and rpm power bands
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Old 12-26-2005, 04:11 PM   #4
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Re: engine running without exhaust

Quote:
Originally Posted by beef_bourito
And what about intake, if you had no intake, just a big tube with one throttle body per cylender, to tuned length. or if you had a diesel with no intake manifold or anything like that, there would be no intake "backpressure".
Lots of upscale, or race cars have separate butterflys per cylinder-- with a manifold used upstream of that to gather clean, cold air (or plumbed to a turbocharger). One of my cars has 4 separate butterflys (4 cyl), and to my knowledge they have never needed adjustment.
I don't understand your second statement, diesels generally don't have throttles on the intake (??).
As for exhausts? You gotta have something to attach the turbo to!
Happy Hanukkah, or whatever!
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Old 12-26-2005, 04:48 PM   #5
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Re: engine running without exhaust

i know that on a diesel there isn't a throttle body so what would be the advantages/disadvantages to just removing the whole intake and having the engine suck straight out of the ports? lets just say that clean air doesn't matter for this question.

And as for the exhaust, lets say it's an open engine bay car so there's nothing to burn and there's not turbo.
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Old 12-26-2005, 06:01 PM   #6
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Re: Re: engine running without exhaust

Quote:
Originally Posted by beef_bourito
i know that on a diesel there isn't a throttle body so what would be the advantages/disadvantages to just removing the whole intake and having the engine suck straight out of the ports? lets just say that clean air doesn't matter for this question.

And as for the exhaust, lets say it's an open engine bay car so there's nothing to burn and there's not turbo.
Had to think about this one awhile even tho I know that people have tried to run VERY short intake pipes on their cars-- the engines had ended up untunable and down on power, even with multi port fuel injection with separate throttle butterflys. They had to revert to a manifold length of some semblance of what the manufacturer originaly had. So I guess the theme of what I think the problem is--is "intake velocity".
So if you have an intake port on lets say, a diesel (no throttle) that simply stops at the head, imagine the intake valve starting to open, the piston going down, air flowing in thru the HOLE, the valve closes, the air stops flowing. End of story. Every intake cycle the scenario plays thru
the same. No intake cycle is connected to the cycle before or after it.
However, if you have a proper length tube on this intake port, the tube acts like a container of sorts. It stores a history of the intake stroke before, the upcoming intake PULSE, and probably the next one to come. In actuality the engine sucks the air in- in a series of pulses, but the air has mass (weight), therefore inertia, so once it is set in motion, it wants to stay in motion (it has a velocity now). The air in the intake pipe is always flowing toward the intake valve because of its inertia. So when the intake valve opens, the air is already crowded against the valve and the engine finds that more air is available to fill the cylinder than if there was no intake pipe, and it had to suck in whatever was available--from scratch, so to speak.
If you can can get this active column of air pulsing and bouncing really well, and the net velocity of the intake charge high enough, the cylinder can be charged over to over 100% efficiency.
An intake port can also contain fuel that is spit out during the valve overlap cycle, so you can see better fuel economy as well.
I'm done.
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Old 12-27-2005, 01:50 AM   #7
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Re: Re: engine running without exhaust

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Originally Posted by beef_bourito
And as for the exhaust, lets say it's an open engine bay car so there's nothing to burn and there's not turbo.
This is generally speaking as there are exceptions to everything:

As was already stated ideally you don’t want any back pressure, it always has a negative impact on performance. However, sometimes back pressure is tolerated to improve the scavenging effects of the exhaust system.

During the exhaust stroke the exhaust gas is pushed out of the cylinder into the exhaust pipe. As the exhaust gas flows through the exhaust pipe it carries momentum. After the exhaust stroke the exhaust valve closes, but the fluid do not stop flowing because of its momentum.

Since the fluid is still flowing through the pipe but no new exhaust gas is being added the exhaust system pressure briefly drops near the exhaust port. Now if the exhaust valve opens again before the temporary drop in pressure disappears it will take less power to drive the next exhaust stroke. Lowering the exhaust system pressure allows the exhaust gases to more readily flow out of the cylinder.

To maximize this savaging effect one needs to maximize the fluid’s momentum. Momentum is dependent on both mass and velocity. Mass is more or less a constant since there is a limited number of fuel sources available, nor would it really be practical to try and change. Velocity on the other hand is directly affected by the exhaust pipe diameter. Larger diameter pipe will have a lower velocity, while smaller diameter pipe will create a higher velocity for a given flow rate.

Since the smaller diameter pipe can impart more momentum to the fluid it would seem to be the better choice for maximum performance. However, the smaller diameter pipe requires more power to maintain the same fluid flow rate. Or another way to look at it is the smaller pipe creates more back pressure.

So a balance between back pressure (pumping losses) and scavenging (fluid inertia) must be found. As well, if the pipe diameter is fixed then it will only be optimized for a single flow rate (read: engine speed). For an engine speed less than ideal the back pressure will be low but the scavenging will also be too low to be useful. At an engine speed above ideal the back pressure will quickly increase nullifying any of the scavenging effects. This allows the exhaust system to be tuned for high-end or low-end power, but not both.

For the case of no exhaust it can theoretically be treated as a pipe with an infinite diameter. So the back pressure would be low, but there would not be any exhaust gas scavenging.
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Old 12-27-2005, 12:35 PM   #8
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Re: engine running without exhaust

True, diesels don't have throttles and sometimes (like my Powerstroke) don't have intakes. They just have a distributing plenum directly into the head ports.

The short answer is that intakes and exhaust are tuned to make proper use of the velocity carried by the mass of the airflow. Adding an exhaust can dramatically increase exhaust flow. I know it sounds backwards, but its not. The tuned exhaust diameter allows the exhaust air to speed up. If you just let the exhaust hit the surrounding air directly out of the ports, it stalls instantly. Putting a tube lets the exhaust achieve speed, which gives it inertia, which carries more exhaust.

Same for intakes. choosing intake runner length and diameter allows the incoming air to get up some steam, so that even after the piston has bottomed out on the intake stroke, the inertia of the incoming air packs in just a little more before the intake valve closes.

In the case of my diesel and other turbocharged engines, it doesn't need the intake's tuning to make proper power because its getting 27 psi from the turbo under almost all conditions
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Old 12-27-2005, 05:36 PM   #9
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Re: Re: engine running without exhaust

Quote:
Originally Posted by curtis73
In the case of my diesel and other turbocharged engines, it doesn't need the intake's tuning to make proper power because its getting 27 psi from the turbo under almost all conditions
Nope, nope, nope. And nobody cares about diesels anyway.
It's true that Billy-Bob can slap a turbo on Bobby-Sue's Mustang and possibly get results that might make him happy without ever removing the manifolds. But on OEM cars and professional racing applications, they spend time getting the inlet and exhaust manifolding correct for off-boost, transition, and boost conditions. Obviously you have to get it a lot more correct if your engine is putting out 1,000 HP from a 1.5L engine, compared to 350 HP on a 5.0L (Bobby-Sue's) engine. The penalty for getting this wrong in both instances is bad drivability and/or a blown engine.
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Old 12-27-2005, 06:15 PM   #10
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Re: Re: Re: engine running without exhaust

Quote:
Originally Posted by Black Lotus
Nope, nope, nope. And nobody cares about diesels anyway.
It's true that Billy-Bob can slap a turbo on Bobby-Sue's Mustang and possibly get results that might make him happy without ever removing the manifolds. But on OEM cars and professional racing applications, they spend time getting the inlet and exhaust manifolding correct for off-boost, transition, and boost conditions. Obviously you have to get it a lot more correct if your engine is putting out 1,000 HP from a 1.5L engine, compared to 350 HP on a 5.0L (Bobby-Sue's) engine. The penalty for getting this wrong in both instances is bad drivability and/or a blown engine.
Curtis was refering to pre turbo intake. Also, the number of 1.5l engines produceing 1000hp (either at teh wheels or crank) is slim, very slim, slimmer than an anerexic etheopian child (one ticket to hell please).

diesels are an entirely different monstor from gasoline engines as well. Where gasoline engines are limited by boost because running lean causes knocking in many cases diesles have no ignition and always run lean (a diesle running rich just needs more boost). basically, making power with diesles is baby's work. as long as you have the internals to handle it, a bottom end that can take it, cooling system that can cool it, oil system that can keep it lubed, and heads that can hold the pressure...
All you need to do is up boost, then up fule, in a continuous cycle untill one of the limiting facots above is reached.
Sure some tuning can make more power on a given set up, however when talking about boosted air doesnt get inertia. Boost is nothing more than air which has nowhere to flow. once the piston is near BDC and pressure is equal on both sides of the valve...no more air is going into that cylender. so intake runner length is just more space for the turbo to have to full with extra air.
However, intake runner length is needed to be tuned to get the engine into a range where it will go on boost. gasiline engines are the ones that really have to worry about this.
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Old 12-27-2005, 06:56 PM   #11
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Re: Re: Re: Re: engine running without exhaust

Quote:
Originally Posted by CBFryman
Curtis was refering to pre turbo intake.
Thanks for the backup, but actually I was referring to the intake manifold You are correct about pre-turbo air intake, and Lotus is correct for off-boost applications in gas engines. Most turbos however are designed for instant boost immediately off throttle so for Gas engines he's right; if you're using a turbo set up for high-rpm boost only. Diesel engines will make tons of their own airflow off-boost, so intake manifold design is somewhat pointless. Most diesels with their small-duration, zero-overlap cams make their peak naturally aspirated VE down around 1200 rpms and boost is usually starting by 1100.

And Lotus... who cares about gasoline? . I just love that you can have a 5.9L diesel that makes 1000 hp and 1500 lb-ft to the wheels, can be driven daily at 26 mpg with 4.10 gears in a 5500-lb vehicle, has a reliability of about 300k miles, and idles like a stocker. That's performance beauty to me. But I guess people these days are happy with only getting the small amount of horsepower that is available from... what's the word again?... gasoline.
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Old 12-27-2005, 11:19 PM   #12
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Re: Re: Re: Re: engine running without exhaust

Quote:
Originally Posted by CBFryman
.
Sure some tuning can make more power on a given set up, however when talking about boosted air doesnt get inertia. Boost is nothing more than air which has nowhere to flow. .
I'm sorry you missed the years of 1.5L turboed F1 cars. Yes F1 cars that were more powerful than some modern day stinky diesels.
I think there is a different way of looking at this. When a manifold plenum is boosted to say, 15 PSI gauge, and to make it simple, lets pretend that all the heat of compression is taken out. You are looking at an intake charge that is double the normal density. Since there is movement in the air column in a running engine (with the throttles open a bit) , there is inertia. In fact, all things being equal, I would guess that there would be a bit more inertia in the intake tract since the intake charge mass is doubled.
At any rate, if you goof up the intake manifold, especially the plenum, you are throwing away drivability and throttle response (you can write that on the stone tablets now).
On a happier note--
Have I ever told you how much stinky, clattery diesels, especially diesel trucks that always tip over, and I can't see around disgust me?
Did you know they stink, clatter and tip over?
Maybe another day.
(Phew) that felt good.
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Old 12-28-2005, 12:26 AM   #13
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Re: engine running without exhaust

modern day diesels aren't stinky, clattery or anything like that, they're very much like gasoline engines in those ways. maybe with the 2 strokes on the big rigs and other very large vehicles.

Diesel is much, much better than gasoline, it's more efficient, it can be made to have more power and more torque than a gasoline engine of the same size, and it's pretty easy, and you can do it without sacrificing fuel efficiency. the only place i can see gasoline type engines being better is in the high reving world of formula 1, even there if there was as much r & d going to diesel as for those gasoline engines, you could have diesels that are alot faster than those. the simple fact is that there's more energy contained in diesel and the engine design is more efficient.

for a street vehicle it's perfect, sports car performance, economy car fuel efficiency, pickup truck towing capabilities, there's no downsides other than some difficulties starting in really cold weather (and that's gotten alot better with newer diesels) so don't diss diesel because you're jealous of all that torque.
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Old 12-28-2005, 01:50 AM   #14
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Re: Re: Re: Re: Re: engine running without exhaust

Quote:
Originally Posted by Black Lotus
Have I ever told you how much stinky, clattery diesels, especially diesel trucks that always tip over, and I can't see around disgust me?
Did you know they stink, clatter and tip over?
Maybe another day.
(Phew) that felt good.
Ha ha. I love a good roast

I'll just show up some day in a popcorn-smelling, biodiesel-burning, E30 and show that little Lotus of your's who's boss

Seriously, your words are wise despite your misguided opinions on the worlds single greatest fuel. In theory the compressed air of which you speak would have more inertia, but the fact that its under pressure makes it less important. If (in your hypothetical example) the intake air charge were doubled in density, it would be doubled in mass, which means for the same intake velocity effect you would basically need half the intake runner length at the same diameter. Since many intake runners are only about 8" long, a 4" runner makes sense for that hypothetical setup. The difference between a 4" runner and no intake at all in the case are neglegible, so that part of the engineering is omitted from its design.

To tie this in with the original question; in the case of turbo exhausts, the pipe diameter after the turbine is often as large as practical. Since the turbine doesn't rely on scavenging or exhaust velocity once its done with it, the best choice for turbo efficiency is often the biggest choice.
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Old 12-28-2005, 04:37 AM   #15
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Re: engine running without exhaust

As someone who has driven a variety of Diesel engines and few of Mr Chapmans creations I can say I would much rather drive the twin carb, single turbo 4 cyl Lotus.
The sort of throttle reponse they can get from a very simple engine lay out (carbs + turbo) is quite amazing, certianly better than any Diesel I have driven, turbo or not.
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